Final answer:
Exposure levels of radiation below the Maximum Permissible Dose are considered safe and reduce, but do not eliminate, the risk of stochastic effects such as cancer and genetic defects.
Step-by-step explanation:
The question pertains to the effects of exposure levels of radiation below the Maximum Permissible Dose (MPD). Exposure levels below MPD are considered safe (option c). This is because, at these levels, there is a reduction in the risk of stochastic effects, but it does not eliminate them (option d). Stochastic effects, which are random in nature, include the long-term effects of radiation such as carcinogenesis and genetic effects. While there is a general assumption that any dose of radiation involves a risk to human health based on the linear hypothesis, the concept of hormesis suggests that low levels of exposure might stimulate protective biological responses.